Update .JuliaFormatter.toml (#2443)

Adding `always_for_in = true` here in general should not be needed, as
`style = "blue"` should already include this, but for some reason my
editor (which admittedly uses an outdated JuliaFormatter.jl) does not
respect this without including this line explicitly.

.JuliaFormatter.toml

@@ -5,3 +5,4 @@ annotate_untyped_fields_with_any = true
 import_to_using = false
 indent = 2
 short_to_long_function_def = false
+always_for_in = true